Gelation mechanism, examination

This paper presents the analysis of the orbital experiment Gel-l , performed on the Mir space station. The use of optical methods enabled us to visualize the photoinitiated reaction of gelation. The examination of obtained specimens has shown that their optical and mechanical properties are more homogeneous than in specimens obtained in earth conditions. To elucidate the role of different gravitational sensitivity mechanisms the data from orbital experiments are compared with the results of laboratory. 

Park and Smith 170 attempted to allow for chain transfer in their examination of the termination mechanism during VC polymerization at 30 and 40 °C in chlorobenzene. They determined the initiator-derived ends in PVC prepared with radiolabeled AIBN and concluded that kjk = 3.0. However, questions have been raised regarding the reliability of these measurements.171 172 Atkinson et al.x72 applied the gelation technique (Section 5.2.2.2) to VC polymerization and proposed that termination involves predominantly combination. 

The compounds we examined in extrusion rheometer and in plastograph had the same recipe as that adopted for determining the physical-mechanical characteristics. However, extrusions were carried out with rigid compounds having an industrial recipe suitable for pipes and sections. Gelation of the dry blend, obtained by turbomixer (10 minutes at 1800 rpm, max. temp. 100°C) was carried out by two-roll mill for 10 minutes at 180°C the blank sheets were mechanically granulated. 

Domain coalescence (Karplus and Weaver, 1976) is a possible mechanism for protein folding. Zientara et al. (1980) examined the dependence of the coalescence lifetime on the hydration shell. The lifetime depends on the activation barrier contributed by the shell and the extent of the shell. If domains resemble the native protein in hydration, then the minimal extent of the shell and its fluidity favor coalescence. In passing, one notes that the percolation model may apply to folding the coalescence of domains should be analogous to gelation or to diffusion on a partially filled lattice. 

Peptide-hydrolyzing activity of Ce(IV) has been examined. In the presence of 10 mM Ce(NH4)2(N03)e, small peptides (10 mM) were hydrolyzed with half-lives of 2h at pH 7.0 and 50°C (106). The reaction was, however, accompanied by formation of hydroxide gels. When the gelation was avoided by solubilizing Ce(IV) with y-cyclodextrin and Gly-Gly (10 vaM) was incubated with the Ce(IV)-cyclodextrin conjugate (10 mA/) at pH 8.0 and 60 C, 39% of the peptide was hydrolyzed after 24 h (107). Coordination of cyclodextrin to Ce(IV) apparently lowered the catalytic activity of the metal center considerably. The mechanism of AU or AV was proposed for the peptide hydrolysis of the Ce(IV) complexes. 

The conformation of acidic polysaccharides and their interactions with calcium ions was examined by molecular simulation, and the authors demonstrated the existence of specific calcium binding with poly-a-L-guluronate [56, 57], The mechanism of complex formation involves calcium interactions with different oxygen atoms of two adjacent guluronic acid units and with two inter-chain units, as visualized in the egg-box model (Fig. 24.7). The mechanism of gelation is a two-step process first step is a dimer formation, followed by precipitation for small chains, or gelation for long ones formed with different types of blocks. 

This work mainly concerns with the question, if there is a correlation in gelation kinetics between network formation and the increasing helical content of the mixture. For this reason we have examined the time dependence of the mechanical parameters G and G" and the specific optical rotation during the gelation process, which has been induced by a temperature jump from 313 K to 293 K. The physical values G, G" and are plotted versus the time for a gelatin/water mixture of... 

The data of Table 17 show the strong reaction enhancements of the specific catalysts impart under microwave heating to all of reactive systems examined. The gelation and vitrification times were lowered to one-eighth to one-tenth of those under hot-air heating with the same catalyst and its concentration. An ion-hopping conduction mechanism was recognized as the dominant source of the microwave absorption capacities of these catalysts [2]. 

The generated inorganic structures highly depend on the hydrolysis and condensation conditions and essentially on the nature of the catalyst used to accelerate the gelation process. The role played by the catalyst in the sol-gel processing of silica has been thoroughly examined by Pope and Mackenzie [43]. On the other hand, the mechanisms and kinetics of hydrolysis-condensation reactions have been widely discussed on the basis of the water content and on the pH dependence of the gelation process [44,45]. 

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